The Multiple Sleep Latency Test (MSLT) is a sleep disorder diagnostic tool. It used to measure the time it takes from the start of a nap period to the first signs of sleep. The test is based on the idea that the sleepier one is the faster they will fall asleep.

It can be used to test for narcolepsy, to distinguish between physical tiredness and true excessive daytime sleepiness, or to see if breathing disorder treatments are working. Its main purpose is to serve as an objective measure of sleepiness.

The test consists of four or five, twenty minute naps that are scheduled about two hours apart. The test is often done following an overnight sleep study. During the test things such as the patient's brain waves, EEG, muscle activity and eye movements are monitored and recorded. The entire test normally takes about 7 hours.

The MSLT may also be used more extensively to test sleepiness in a number of other research and diagnostic protocols.

The Multiple Sleep Latency Test was created in 1975 by sleep pioneer William C. Dement and Mary Carskadon. It developed out of repeating a project done in 1970 by Dr. Dement called the 90-minute day. They informally called the 0-5 score range the twilight zone due to its indication of extreme physical and mental impairment.